A. Field of the invention
The present invention relates to linearization circuits in fiber optical communications systems and more particularly to providing distortion compensation for third-, fifth- and seventh-order intermodulation and harmonic distortion components of single-stage and dual-stage serial Mach-Zehnder modulators.
B. Statement of Related Art
Optical communications systems adapted to carry a wide range of information including voice, video and data are well known in the art. The typical optical communications system includes an optical modulator or a laser transmitter which transduces an electrical information signal into an optical signal. The optical signal is then carried over an optical fiber communications link where it is converted back to an electrical signal by a photodetector of an optical receiver. The transmission scheme may be analog or digital and the modulation scheme amplitude, phase, frequency, or any combination of the above.
Particularly within the field of optical communications, the use of Mach-Zehnder optical modulators are well-known. The Mach-Zehnder optical modulator mixes a radio frequency (RF) information-bearing signal with a lightwave carrier by electromagnetic phase interferometry. Upon entering the Mach-Zehnder optical modulator, the lightwave carrier is typically split into two signals that are coupled into separate waveguides formed in the crystal structure of the modulator. Electrodes are placed in close proximity to the waveguides in the device. An RF information-bearing signal is applied to the electrodes next to one of the waveguides. The propagation of the lightwave carrier through the waveguides is affected by electric field variations that the RF signal causes in the propagation characteristic of the waveguide in the area near the electrodes. The electric field causes a local change in the refractive indices around the waveguides, thereby speeding up the propagation of the wave in one path while delaying the other. The relative phase of the two lightwave signals in the modulator is changed in proportion to the modulating signal applied to the electrodes. At the output of the Mach-Zehnder optical modulator, the phase modulated optical carrier signals are recombined. When the two optical signals having variations in relative phase are recombined, phase interference occurs which can be destructive and/or constructive. The result is a modulated lightwave output having amplitude changes in proportion to the modulating RF signal. The modulated lightwave output can then be coupled to a fiber optic medium for transmission over long distances.
The optical modulation provided by Mach-Zehnder optical modulators is generally superior to that of direct laser modulators. One advantage with the Mach-Zehnder optical modulator is that it is not subject to "chirp" (residual frequency modulation) where the spectrum of the optical beam is broadened. Other advantages are that Mach-Zehnder optical modulators have wide RF bandwidths and are memoryless devices with predictable distortion profiles. The advantages of using optical modulators versus direct laser modulators in terms of non-linear intermodulation distortion are discussed further by G. E. Bodeep and T. Darice, "Comparison of second and third order distortion in intensity modulated InGaAsP lasers and an LiNbO.sub.3 external modulator", Paper WK2, OFC '89 Conference on Optical Fiber Communications, Houston, Tex., February 1989, and is incorporated herein by reference.
However, optical modulators, like their semiconductor counterparts in RF electronics, are non-linear devices. The typical Mach-Zehnder optical modulator consists of a lithium niobate (LiNbO.sub.3) crystal device having linear and non-linear modulation characteristics. Any deviation of the modulator transfer function from the linear range of the modulator causes odd- and even-order harmonic distortion. Therefore, in order to optimize the quality of the modulated output from an electro-optical modulator, it is desirable to apply a bias control to the device to set its operating point, or bias point, as close as possible to the center of its linear range. Since the principle of operation of the Mach-Zehnder optical modulator is phase interferometry, the center bias point is very sensitive to temperature, input signal fluctuations, and manufacturing tolerances. If not properly biased, the modulator will generate even-order harmonic distortions in addition to odd-order harmonic distortions. These distortions degrade signal quality, thereby reducing the dynamic range of the system.
Recently, there has been a growing interest in the use of Mach-Zehnder optical modulators for use in high power amplitude modulated vestigial sideband (AM-VSB) CATV fiber optic supertrunk and distribution systems for long-haul transmission. However, these systems require high performance (high signal modulation depth) fiber optic broadcast channels to overcome the noise accumulated by fiber amplifiers placed along the transmission path. However, as the signal modulation depth is increased, distortion increases rapidly. The system cannot tolerate higher-order harmonic distortions such as third-, fifth- and seventh-order distortion components. Even-order distortion components of the Mach-Zehnder optical modulator may be suppressed by properly adjusting the electrical bias level, leaving only the odd-order distortion components as the limiting factor in achieving a linear fiber optic system.
The transfer function for the output power of the Mach-Zehnder optical modulator has a predictable distortion profile of a well-known sinusoidal function. A number of techniques are known in the art for suppression of third-order distortion components. For instance, third-order distortion components can be suppressed in a single-stage Mach-Zehnder optical modulator by electrically predistorting the multichannel signals. Third order distortion components can also be eliminated in a dual-stage cascaded electro-optic modulator by driving the two modulator electrodes with an RF signal split in two in-phase paths with properly adjusted magnitudes. These methods are described by U.S. Pat. Nos. 5,249,243 and 5,278,923. Moreover, U.S. Pat. No. 5,327,279 discloses a number of other methods known in the art for improving linearization of Mach-Zehnder optical modulators.
Predistortion techniques have generally been favored over other linearizing techniques because of its relatively wideband characteristics and ability to function in stand-alone units. A predistortion circuit distorts a modulating signal equally in amplitude and frequency but phase shifted 180.degree. with respect to the transfer function of the optic modulator before feeding the modulating signal to the optical modulator. Thus, the predistortion effectively cancels the distortion produced by the optical modulator. A predistortion circuit, in theory, would employ a simple diode network to synthesize the transfer function arc sin (x). The transfer function arc sin (x) is used since the transfer function of the optic modulator is sin (x). Thus, when the signal is optically modulated, the distortion components of the resulting optical signal are thereby canceled (since sin arc sin (x)!=x).
However, predistortion circuits in the prior art have only been able to approximate an ideal predistortion circuit. As a result, the present state of the art cannot provide higher optical modulation depths necessary for transmitting high performance AM-VSB CATV fiber optic broadcast channels.
It is therefore an object of the present invention to provide improved linearity through providing improved distortion of the RF signal in an optical system to accommodate transmission of high performance AM-VSB CATV signals.
Another object of the present invention is to provide improved linearity by distorting the RF signal in an optical system to accommodate transmission of high performance AM-VSB CATV signals.
Yet another object of the present invention is to provide a distortion circuit for linearizing an externally modulated optical modulator.
Still another object is to provide a distortion circuit which generates higher-order harmonic distortion components to cancel non-linearities of Mach-Zehnder optical modulators.